The present invention relates to a computer display apparatus having a composite video interface driven by a power supply which drives a processor section.
A conventional image display apparatus of the type included in computer display apparatus has a composite video interface for handling video or audio analog signals and is driven by a power source which drives a processor section. In this image display apparatus, switching noise from the processor section, a switching regulator or the like is inserted in the composite video interface through a power supply line, so that jitter occurs in an output image or sound, thus degrading the output image or sound quality. FIG. 1 shows part of a conventional computer image display apparatus, and FIG. 2 shows a waveform of an output signal therefrom. Referring to FIG. 1, a video composite video interface (to be referred to as an interface for brevity hereinafter) serves as a circuit for generating a monochrome composite signal. This interface is connected to a central processing unit (CPU) 10, a key input section and a memory. Signal lines 11, 12 and 13 for transmitting a 3-bit signal representing 8-level gradation from a white level to a black level, and a signal line 14 for transmitting horizontal (H) and vertical (V) sync signals are connected to the processor section (CPU) 10. The signals on the signal lines 11, 12, 13 and 14 are supplied to an interface 20 which then generates a monochrome composite signal. The interface 20 has resistors R1 to R8, a driving transistor Q1 and monochrome composite signal output terminals 15 and 16. The monochrome composite signals are supplied to a CRT monitor 17 through the output terminals 15 and 16. A power supply 30 commonly supplies driving power (VCC, 5 V) to the CPU 10 and the interface 20 through a power supply line 31.
In this computer image display apparatus, the respective logic signals of TTL level supplied from the CPU 10 onto the signal lines 11, 12, 13 and 14 are combined by the resistors R1 through R6 of the interface 20 and are converted to an analog signal. This analog signal is applied to the base of the transistor Q1. Since the transistor Q1 constitutes an emitter follower, a voltage drop between the base and emitter thereof appears as an emitter output at the output terminals 15 and 16. The monochrome composite signals generated from the output terminals 15 and 16 comprise a signal Ci which is obtained as a combination of the bit signals on the signal lines 11, 12 and 13 and which represents a given luminance of the 8-level gradient varying from the white (W) level to the black (B) level, and horizontal and vertical sync signals (H and V), as shown in FIG. 2. This conventional composite video interface is described in "D-31: the IBM Personal Computer Technical Reference Manual" 6936895, First Edition, January 1983. Color gradation is described in "HOISTING THE COLOR STANDARD" by David H. Stvaayer, "COMPUTER DESIGN" July 1982, pp. 123-130.
When a switching noise component generated from the CPU 10 or a switching circuit of the power supply 30 runs on the VCC power supply line 31, as shown in FIG. 3A, this noise component passes through the resistor R5 and the transistor Q1 and appears at the output terminals 15 and 16. In this case, an output waveform is illustrated in FIG. 3B, wherein switching noise N runs on the VCC power supply line 31 when an intermediate-level signal appears at the output terminals 15 and 16. When the monochrome composite signal including this noise N is supplied to a CRT monitor 17, the noise N is visually displayed as interference in an image of an intermediate color. In general, the switching noise component generated from the CPU 10 and the power supply 30 has a high frequency and is continuously generated at a predetermined interval, so that stripe-like and flickering noise occur on the screen, thus degrading image quality. In particular, when the noise component has substantially the same frequency as that of the horizontal sync signal, beat noise occurs.